i) Field of the Invention
The invention relates to a process for control of the free lime content of cement clinker, more especially for elevating the normal free lime content of cement clinker.
ii) Description of Prior Art
In the production of Portland cements, the raw ingredients pass through different heating stages in a rotary kiln. In the kiln the ingredients pass through a calcining zone in which calcium carbonate is thermally decomposed to lime; the calcining zone is typically at a temperature of 825.degree. C. to 1000.degree. C. Thereafter the calcined material enters a burning zone which is at a temperature typically of 1375.degree. to 1450.degree. C.; the chemical reactions which form the cement clinker take place in the burning zone.
The formed cement clinker passes through an initial cooling zone downstream of the burning zone and thence into a cooler where the temperature profile gradually drops.
The free lime content of cement clinker can more or less be controlled in the production of the cement clinker, but varies from installation to installation and as between different types of installation, for example, precalciner process kilns and wet process kilns. In some operations the cement clinker produced has a free lime content of less than 0.5%, by weight.
In general, while it was known previously that cement clinker produced by some conventional processes had a free lime content often as low as about 0.25%, this was not a problem because the strength was satisfactory. Furthermore, the process operating parameters in the manufacture of cement clinker, for a particular rotary kiln, are fairly rigidly controlled and variations in such parameters are generally avoided. Altering the ratio of the components of the raw ingredients, or process parameters such as the temperature of the different zones, or the rate of flow through the kiln, which affects the holding time of the materials in each zone, disturbs the desired clinker formation.
Thus, in general, the process operating parameters in cement clinker manufacture, for a given kiln, are well established, are carefully controlled to ensure the desired chemical reactions for the production of a clinker of desired characteristics, and there is strong resistance to altering these parameters for any reason, because such alteration affects the clinker production and the characteristics of the clinker adversely.
Thus even though there is a desire to control the free lime content of cement clinker to a particular level, persons in the art would not have considered altering the established process parameters for clinker production in a particular kiln since such alterations could affect the stability of the operation.